1. Field of the Invention
The present invention relates to a solid-state imaging device, and particularly relates to a CCD (charge coupled device) type solid-state imaging device.
2. Description of Related Art
Recently, due to the rapid prevalence of digital cameras, digital movie cameras and mobile phones equipped with camera functions, the demands for solid-state imaging devices have been increased rapidly. In particular, in accordance with the demand for the increase of the number of pixels and functions for supporting moving images, the increase of a driving speed of solid-state imaging devices has been desired recently. As one measure for this, for example, JP 3(1991)-256359 A discloses a shunt wiring structure in which a shade film also functions of supplying a transfer pulse to a transfer electrode of a vertical CCD.
A CCD type solid-state imaging device having a shunt wiring structure will be described with reference to FIGS. 24 to 28. Initially, an entire configuration of the solid-state imaging device will be described. FIG. 24 is a structural view schematically showing an entire configuration of a conventional solid-state imaging device having a shunt wiring structure. As shown in FIG. 24, the solid-state imaging device has a semiconductor substrate 101 that is provided with a plurality of pixels 104. The pixels 104 are arranged in matrix, and a region where the pixels 104 are arranged is an image formation region 101a. In this example, a n-type silicone substrate is used as the semiconductor substrate 101 (see FIG. 26).
Each of the pixels 104 has a vertical charge transfer part (vertical CCD (charge coupled device)) 102 and a photodiode part 111. Moreover, on the semiconductor substrate 101, a horizontal charge transfer part (horizontal CCD) 103 is formed along a horizontal direction on a side of a transfer direction of the vertical CCD 102. At an output end of the horizontal CCD 103, an output amplifier 103b is provided. An arrow in FIG. 24 represents a transfer direction of electric charges.
Each of the vertical CCD 102 and the horizontal CCD 103 has a transfer channel that is formed on the semiconductor substrate 101, and a first transfer electrode and a second transfer electrode that are disposed on the transfer channel, which are not illustrated in FIG. 24. It should be noted that, in the present specification, the transfer electrode of the vertical CCD is called a “vertical transfer electrode”, and the transfer electrode of the horizontal CCD is called a “horizontal transfer electrode”.
In a peripheral region of the image formation region 101a, a vertical bus line part 116 is provided along an outer periphery of the image formation region 101a. The vertical bus line part 116 has a vertical bus line wirings 116a to 116d. To the respective vertical bus line wirings 116a to 116d, different transfer pulses ΦV1 to ΦV4 are supplied externally.
Further, a horizontal bus line part 117 is provided along the horizontal CCD 103. The horizontal bus line part 117 has horizontal bus line wirings 117a and 117b. Also to the respective horizontal bus line wirings 117a and 117b, different transfer pulses ΦH1 and ΦH2 are supplied externally.
Moreover, a stripe-shaped shade film 113 is formed on each vertical CCD 102 so as to cover the vertical CCD 102 such that light is not incident into the vertical CCD 102. The shade film 113 is made of a metal material, and is connected to either of the vertical bus line wirings 116a to 116d. Moreover, the shade film 113 is connected to the vertical transfer electrode via a contact hole 114. Thus, the shade film 113 functions as a shunt wiring that supplies a transfer pulse to the vertical transfer electrode of the vertical CCD 102. It should be noted that a part of the shade film 113 is omitted in FIG. 24.
Next, a configuration of the pixel of the solid-state imaging device shown in FIG. 24 will be described specifically with reference to FIGS. 25 to 28. FIG. 25 is a plan view showing a configuration of a pixel and its periphery of the solid-state imaging device shown in FIG. 24. FIG. 26 is a cross-sectional view taken on line X-X′ of FIG. 25. FIG. 27 is a cross-sectional view taken on line Y-Y′ of FIG. 25. FIG. 28 is a cross-sectional view taken on line Z-Z′ of FIG. 25. Incidentally, the shade film 113 is shown in broken line in FIG. 25. In FIGS. 26 to 28, members that are made of metal materials (except for the semiconductor substrate) are hatched.
As shown in FIGS. 25 and 26, the vertical CCD 102 has a transfer channel 102a, a first vertical transfer electrode 106 and a second vertical transfer electrode 109. A plurality of the transfer channels 102a are formed along respective vertical columns of a plurality of the photodiode parts 111 in a surface layer of the semiconductor substrate 101.
As shown in FIG. 25, the first vertical transfer electrode 106 and the second vertical transfer electrode 109 are formed above the plurality of the transfer channels 102a via a gate insulation film 105 (see FIGS. 26 to 28), so as to cross the plurality of the transfer channels 102a. The shade film 113 is formed in stripe so as to cover the vertical CCD 102. The shade film 113 is connected to the first vertical transfer electrode 106 via a contact hole 114a (see FIG. 27). Moreover, the shade film 113 is connected to the second vertical transfer electrode 109 via a contact hole 114b. 
As shown in FIG. 26, the photodiode part 111 has a photoelectric conversion region 111a formed of a n-type diffusion region, and an inversion layer 111d formed above the photoelectric conversion region 111a. Moreover, between the photodiode part 111 and its corresponding vertical CCD 102, a read-out region 111b is formed. A signal charge formed in the photoelectric conversion region 111a is read out to the transfer channel 102a via the read-out region 111b. The read-out region 111b is a p-type diffusion region. Moreover, a pixel isolation region 111c is formed between the photodiode part 111 and a vertical CCD that corresponds to another photodiode part 111 so as to isolate them. The pixel isolation region 111c is a p-type diffusion region that has an impurity concentration higher than that of the read-out region 111b. 
Moreover, as shown in FIG. 27, above the transfer channel 102a, the first vertical transfer electrode 106 and the second vertical transfer electrode 109 are arranged adjacent to each other so as to transfer electric charges. Further, the second vertical transfer electrode 109 is formed so that an end thereof is overlapped with an end of the first vertical transfer electrode 106. Whereas, as shown in FIG. 28, on a region where the transfer channel 102a is not formed, the second vertical transfer electrode 109 is disposed on the first vertical transfer electrode 106.
Moreover, in FIGS. 27 and 28, reference numeral 108 denotes an insulation film that is formed on an upper surface and lateral surfaces of the first vertical transfer electrode 106 by thermal oxidation. Reference numeral 112 denotes an insulation film that is formed on an upper surface and lateral surfaces of the second vertical transfer electrode 109 by thermal oxidation. Reference numeral 115 denotes an insulation layer (shade film insulation layer) for insulating the first vertical transfer electrode 106 and the second vertical transfer electrode 109 from the shade film 113. Moreover, as shown in FIG. 27, the contact hole 114a that connects the shade film 113 and the first vertical transfer electrode 106 is formed so as to pierce the insulation film 108 and the shade film insulation layer 115.
Further, an insulation layer 118 is formed so as to cover an entire upper surface of the semiconductor substrate 101 including the shade film 113. The vertical bus line part 116 shown in FIG. 24 is formed on the insulation layer 118, which is not illustrated. Incidentally, the illustration of the insulation layer 118 is omitted in FIG. 26.
Operations of the solid-state imaging device shown in FIGS. 24 to 28 will be described. In the below explanation, FIGS. 24 to 28 will be referred to, as appropriate. Initially, when an optical image is formed on the image formation region 101a of the semiconductor substrate 101, the photoelectric conversion region 111a of each photodiode part 111 performs photoelectric conversion, and accumulates a signal charge according to an intensity of the incident light and an incident time.
In this state, when a high-level voltage VH (10 V to 15 V) is applied to the second vertical transfer electrode 109 via the vertical bus line part 116 and the shade film 113, a potential directly underneath the second vertical transfer electrode 109 becomes high. Thereby, the signal charge accumulated in the photoelectric conversion region 111a of each photodiode part 111 is transferred to the transfer channel 102a of the vertical CCD 102 through the read-out region 111b. 
For example, in the example shown in FIG. 25, the second vertical transfer electrode 109 at a center of the figure is connected to the shade film 113 on a right side of the figure via the contact hole 114b. Thus, when the high-level voltage VH is supplied to the shade film 113 on the right side of the figure, the high-level voltage VH is applied to the second vertical transfer electrode 109 at the center of the figure. As a result, all of the photoelectric conversion regions 111a shown in FIG. 26 start reading out the accumulated signal charges.
Next, a middle-level volgate VM (0 V) and a low-level voltage VL (−5 V to −10 V) are applied alternately to the first vertical transfer electrode 106 and the second vertical transfer electrode 109. Thereby, the signal charges are transferred sequentially in the vertical direction, and reach the horizontal CCD 103.
Thereafter, a high-level voltage (2 V to 5 V) and a low-level voltage (0 V) are applied alternately to a first horizontal transfer electrode (not illustrated) and a second horizontal transfer electrode (not illustrated) of the horizontal CCD 103, via the horizontal bus line part 117. Thereby, the signal charge is transferred from the horizontal CCD 103 to the output amplifier 103b. 
The output amplifier 103b converts the signal charge into a voltage, and outputs the signal voltage to the outside. As described above, the signal charge accumulated in the photoelectric conversion region 111a is transferred in the vertical direction by the vertical CCD 102, and is transferred in the horizontal direction by the horizontal CCD 103, thereafter being output to the outside.
By the way, in the solid-state imaging device having the shunt wiring structure, when a voltage is applied to the shade film 113, an electric field is generated thereby. Then, the photodiode part 111 is influenced by the electric field generated by the shade film 113 that is positioned on a read-out side. More specifically, this electric field is stronger as the voltage applied to the shade film 113 is higher. And, as the electric field is stronger, a read-out path from the photodiode part 111 to the vertical CCD 102 is nearer to an interface. As a result, the number of electrons that are trapped in an interface state is increased at the time of the read out, and an output value from this photodiode part is decreased.
For example, in FIG. 26, since the voltage VH is supplied to the shade film 113 on the right side of the figure, the shade film 113 on the right side of the figure generates a strong electric field. Thus, as shown in FIG. 26, in the photodiode part 111 on the right side of the figure, where the shade film 113 on the right side of the figure is positioned on the read-out side, the read-out path is near to the interface, and many electrons are trapped in the interface state.
Whereas, in FIG. 26, since the voltage VL is applied to the shade film 113 on the left side of the figure, an electric field generated by the shade film 113 on the left side of the figure has a polarity opposite to a polarity of the electric field that is generated by the shade film 113 on the right side of the figure. Thus, as shown in FIG. 26, the read-out path of the photodiode part 111 at the center of the figure is positioned deeper than the photodiode part 111 on the right side of the figure, and the number of electrons that are trapped in the interface state also is smaller.
As a result, the output value from the photodiode part 111 at the center of the figure is larger than the output value from the photodiode part 111 on the right side of the figure, and unwanted vertical stripes are displayed on a display, thereby degrading the image quality of a shot image.